Low alloy-air hardening die steel



Feb 12, 1963 P. R. BORNEMAN Low ALLOY-AIR HARDENING DIE STEEL 2Sheets-Sheet 1 Filed May 9, 1961 Fig Fig.4

Im mm 6 Feb.- 12, 1963 P. R. BORNEMAN 3,077,397

LOW ALLOY-AIR HARDENING DIE STEEL Filed May 9, l1961 2 Sheets-Sheet 210005 l l 'l l 200 400 600 SOO |000 |200 TEMPERING TEMPERATURE- F3,077,397 Patented Feb. 12, 19163 3,077,397 LQW ALLGY-All HARDENNG DEESTEEL Paul R. Borneman, Natrona Heights, Pa., assigner to AlleghenyLudlum Steel Corporation, Breckenridge, Pa., a corporation ofPennsylvania Filed May 9, 1961, Ser. No. 103,869 4 Claims. (Cl. 75126)This invention relates to improvements in die steels, and relates inparticular to a new and novel low alloyair hardening die steel.

The low alloy-air hardening die steel to which the present invention isdirected, is conventionally used for tools and dies having a design thatprohibits the use of water hardening steels because of the hazard ofdistortion or cracking during hardening. The oil hardening steelsgenerally hold their dimensions less closely than the air hardeningcompositions. rEhe requirements of such steels are a combination or abalance of deep hardening characteristics, a Wide hardening range, goodimpact resistance and good machinability in addition to dimensionalstability during heat treatment. The aforementioned optimum propertiesare desired and required, particularly for the manufacture of dies usedin punching, coining, piercing, blanking, stamping and trimming ofmetals and are also useful in the manufacture of hubs, bushings .andmaster tools where high dimensional tolerances are of importance. Thelow alloy-air hardenable `steels now available commercially satisfyseveral of the aforementioned properties; however, none satisfies all ofthe property requirements in good balance.

A die or tool steel has now been discovered which exhibits deephardening characteristics, a wide hardening "range, good impactresistance, and is easily machined in addition to exhibiting unusuallygoed dimensional stability.

' In general, the present invention is directed to .a steel containingmore than about .80% but less than .90% carbon, from 1.85% to 2.15%manganese, .25% to .50% silicon, .75% to 1.l% chromium, from 1.20% to1.40% .molybdenum and from about .45% to .85% vanadium, the balancebeing essentially all iron. The carbon content preferably falls withinthe range of from about .83%

to .87%, and sulfur is optionally present within the range of rrom about.08% to .10% to impart improved machinability.

Consequently, it is an obiect of the present invention to provide a lowalloy-air hardenable die or tool steel that exhibits good deepzhardening characteristics, a wide hardening range, good impactresistance, good machinability in the annealed condition and gooddimensional stability during heat treatment.

It is also an object of the present invention to provide a die or toolsteel which exhibits superior dimensional stability during heattreatment to prior known low alloy deep hardening compositions.

It is a further object of the present invention to provide a die or toolsteel which exhibits a wider hardening `range than the prior known lowalloy deep hardening compositions.

Other objects and advantageous features of the present invention will beobvious from the following description and the accompanying drawingswherein:

FIGURE l is a graphical representation of the dimensional distortionwhich occurs in the steel of the present invention .as compared withsimilar prior known compositions;

FIG. 2 is a graphical representation of the comparative dimensionalstability properties of analyses that constitute another embodiment ofthe present invention; and

FIGS. 3 and 4 are cross-sectional views of 4" and 6"' cubes,respectively, showing hardness measurements taken from the center to theedge which illustrate the hardenability of the alloy of the presentinvention. In quenching and tempering low alloy-air hardeningcompositions, it has been determined that the dimensional changes takeplace upon phase transformation and the dimensional changes areparticularly noticeable during subsequent tempering heat treatments. Theparts or dies are conventionally machined in the softer annealedcondition and are then heated andquenched to effect maximum hardness fortheir ultimate use which demands such properties. During heating,transformation of the structure of the material to a phase known asaustenite is .accompanied by a decrease in size. Upon quenching theaustenite is transformed to martensite; however, some of the austenitefails to transform and is referred to as retained austenite. Temperingis conducted in order to complete the transformation of retainedaustenite into martensite or bainite and to relieve stresses that mayresult in subsequent cracking. This transformation is known to generallyresult in an expansion of the metal and unless the shrinkage whichoccurs during heating is equivalent to such expansion, .a distortion ofthe die or tool occurs. The resulting die or tool steel part which hasbeen shaped to close tolerances, is frequently found to be unacceptable.The tempering temperatures Vary from room temperature to 1000 F., butgenerally occur between the temperature range of from about 400 F. to650 F.

In the composition of the present invention, manganese, chromium andmolybdenum are present as hardening agents Within the usual and knownranges which are relatively critical in providing optimum deep hardeningcharacteristics and impact resistance. The carbon content is alsopresent as an essential hardening agent, since carbon, .as is wellknown, is a necessary material for the formation of martensite which isthe primary hardening phase. ln the composition of the presentinvention, however, the carbon is present within critical limitations inthat the amount of carbon present must exceed .80% (about .83%) in orderto impart the necessary hardening and hardenable characteristics, While4it must be below .90% (about .87%) in order to retain the excellentstability of the steel. Vanadium also must be within the critical rangelimitations of from about .45% to .85%. The function of the vanadiumover and above .about .20%, which is dissolved in the steel and acts asa hardening agent, is beieved to be in combining with the carbon. lfexcessive amounts of vanadium are used, the effects are identical tothat of insufficient amounts of carbon, in that insuicient amounts ofavailable carbon caused either by excessive vanadium or too littlecarbon in the composition would result in insuicient hardeningproperties. On the other hand, the vanadium carbide precipitates atgrain boundary junctions, and having a high solution temperature,controls grain growth and contributes to a wide hardening temperaturerange. This same vanadium carbide in some Way appears to contribute tothe dimensional stability, probably by controlling the amount of carbonwhich can be taken into solution during austenitizing. Consequently, ifthe vanadium is too low or below the critical range set forth, thedimensional stability properties are not obtained. i

TABLE I Designation C Mn Si Cr Mo V S T 2.14 36 1.06 1.35 .03 XT-OZSEZ2.02 24 1.02 1. 22 .02 .081

The superior dimensional stability of applicants alloys as compared tothe prior known similar compositions is particularly illustrated by thedata shown by the following Tables II, III and IV and FIGS. 1 and 2 ofthe drawings. In Table II, there are shown the dimensional changeseffected on 3%: inch round X 2.000 long specimens of the materialdesignated in Table I as .AL-123, Airloy and XT-O28 when the material ishardened and hardened plus tempered at temperatures ranging up to 1000F. Each of the commercial grades was hardened by air quenching from thetemperature recommended for the specific grade to secure optimummechanical properties and applicants composition was quenched from 1525F., which is within 25 F. of the quenching temperature of the othermaterials and which temperature gave applicants composition equivalentor superior mechanical properties as well as superior dimensionalproperties. The dimensional changes were determined by measuring thelength of the bars before and after heat treatment. The data of Table IIis plotted graphically in FIG. 1 to more clearly show the advanages ofapplicants analysis. In FIG. l dimensional changes are plotted againsttempering temperatures and curve 1 represents applicants alloy, whilecurve 2 shows the dimensional stability of the XT-028 analysis and thecurve 3 the Airloy composition. The line 0 represents no dimensionalchange and the initial plots indicate the dimensional change occurringupon quenching.

TABLE II Dimensional Stability [Average change inches/inch of length 0i"round x 2.0000" I'As hardened, change from the finish machined size.NOTE 1.-- l- Denotes expansion, denotes contraction.

NOTE 2.-XT028-Hardened at 1550F. for 8 minutes TAT-Air Quench.AL-123-Hardened at 1525F for 8 minutes TAT-Air Quench. Airloy-Hardenedat 1500F for 8 minutes TAT-Air Quench.

It may be readily observed from either Table II or d FIG. 1 thatapplicants alloy exhibits less dimensional change either as quenched oras quenched and tempered at any tempering treatment up to l000 F.Although greater dimensional change is experienced by all three alloysin the 450 F.600 F. temperature range, applicants alloy exhibits theleast change or deviation from the original dimensions of the bars.

Table III shows the dimensional properties of 1A round samples of thesame materials employed to obtain the data shown by Table II and FIG. l.This data confirms the data of Table II in that in nearly everyinstance, applicants alloy shows far greater dimensional stability thanthe other grades.

TABLE III [y round x 2.0000 long samples- Heat treatment identical to3/4" rd. samples] Temper- XT-028 .AL-123 Airloy ing oteFmp.,

+. 00005 00005 00050 (l) 00018 00005 -i-Y 00048 200 00051 .00005 l.00037 300 00066 .00008 +.00020 400 00083 +.00020 .00000 500 00007+.00051 +.00080 600 00060 .00012 00110 700 00081 00031 -l-.00090 80000089 00045 00070 900 00103 00055 -f-.00060 1,000

1 As hardened.

In Table lV and FIG. 2, there is shown dimensional stability ofapplicants alloy AL-123EZ for diierent quenching temperatures. FIG. 2and Table lV also show the comparative dimensional stability of asimilar grade identified as XT-028EZ (see Table I). These analysescontain sulfur that has been added to improve machinability. %-inchround x 2.000 long samples were hardened at the recommended temperaturesfor the respective materials and at temperatures 50 F. above and 50 F.below these temperatures for 8 minutes and air quenched. The specimenswere measured (lengthwise) to the fourth decimal place before hardening,after hardening and after each temper. The results were as follows:

TABLE IV Dimensional Stability Average Change [Inches/inch of length 27ground x 2.000" long samples] AL-123EZ XT-OZSEZ Austenitizingtemperatures, F.

1 Dimentional changes are in inches/in.

In FIG. 2 the plots represent the actual dimensional changes as setforth in Table IV. The graph more clearly shows the advantages ofapplicants analyses. All plots designated A, A' or A are plots ofanalyses reported in Table I above (AL-IZBEZ), which are within thescope of the present invention, while al1 plots designated X,

TABLE V Hardenng and T emperng Data [Samples 2 long x 135 rd. of steelhaving the analysis shown by Table I, were hardened lor 5 minutes at theindicated temperature and air cooled. They were fractured and tested forhardness. The samples were then tempered cumulatively for lhour at theindicated temperatures with the Rockwell C hardness checked aft-er cachtemper] t Grain size Cumulative 1-hour draw at- Hardening temp.,Hardness Shepherd F as quenche( rating 300 F. 400 F 500 F. 600 F. 700 F.800 F. 900 F. 1,000 F 1,350 26. 5 5 27 29 28 28 27 23 25 26 1,400. 52. 57% 54 5 l 54 53 52 52 50. 5 49 1,450- 57 9% 58 57 56 56 53. 5 l 53 5l1,50 59. 5 l0 61 59 57 56. 5 53. 5 54. 5 54 52 1,550- 59. 5 l0 60.5 5956 55. 5 55 56 55. 5 52 1,600 52. 5 l0 62 60 59 57 57 56 52. 5 1,65 63.5 9% 62. 5 69. 5 57 57. 5 55. 5 56 55 54 1,70 63 10 62 60 57 56. 5 56. 555 55 54 1,75 63 l0 63 69. 5 58 57 56 55. 5 55 53. 5 1,80 63. 5 l0 63 6257 57 56 55 55 54 1,8 62. 5 9 62. 5 60. 5 57. 5 57 56 55. 5 55 54 1,9063 9 63. 5 60 57 57 55 55 56 54. 5 1,95 63 9 63 59. 5 57. 5 55. 5 56 5555 54 2,0 02. 5 8 62. 5 59. 5 58 57 57 56 54 55 X or X are plots of theXT-028EZ material. The temperatures from which the samples were quenchedvaried, each material being air quenched from the temperature from whichoptimum die steel properties are obtained and at 50 F. above and belowsuch optimum tempera- As may be readily seen from the data of Table IVand from FIG. 2 at any given temper applicants analysis exhibits farless distortion than the presently available compositions. This datashows that the degree of stability of applicants alloy varies to someextent in accordance with the hardening treatment, but that similarcompositions also vary, -but are generally less dimensionally stableregardless of the exact head treatment given.

As may be observed from the data of Tables II-IV and FIGURES l and 2,applicants compositions exhibit a closer relation to the theoreticalzero limit of dimensional change than other low alloy-air hardening diesteels now being used. This holds true for both large and small samplesize showing that the vanadium addition in combination with the properlevel of carbon produces close dimensional stability.

FIGS. 3 and 4 demonstrate the hardening properties of applicants alloy.These figures show the results obtained when 4" and 6 cubes ofapplicants alloy (AL- 123) were heated to l550 F., air cooled and cnt inhalf, then tested for Rockwell C hardness from the edge and corner ofeach to the center of the former cubes. The results, as shown, clearlyillustrate the good deep hardening characteristics of the material.These characteristics are known to be prevalent over la hardening rangeof l500 F. to 1800 F. Such a wide band of acceptable hardeningtemperatures is greatly advantageous to a fabricator, since he can use avariety of equipment which may be available to him for such heattreatment and, additionally, can heat treat the material for purposes ofquenching with other steels of various grades.

Table V shows the wide hardening range of the .AL- 123 analysis. Sampleswhich were air quenched at temperatures of from 1500 F. to 1800 F. areshown to pos- Table VI below, shows the Izod impact properties of thehardened and quenched AL123 analysis. These properties are shown to beequivalent or superior to those possessed by the known commerciallyavailable materials.

TABLE VI Izod Impact Tests at Room Temperature [Unnotched Izod samples 3long x .425 square were hardened at the indie :teil temperatures and airquenched. Tempering for 1 hour at the indicatedtempcraturepreccdedfinish grinding to .304" squares. The samples were then tested in a it.lb. Izod machine] Hardening Tcmpering Hardness Average temp., F temp., FRockwell C impact value, ft. lbs.

1, 500 (l) 13 51.75 1, 500 300 42 62 1, 500 400 39 -flO 71 1,500 500 4282 1. 500 600 40 1, 600 (1) h3 27. 50 1, 600 300 6l -62 55 1, 600 400 5968. 25 1 500 500 50. 557 5 S2 1, 0 600 55 -50 76.50 1,800 (1) b2 -63 23.25 1,800 300 t0 -01 57.25 1,800 400 5S -59 89.50 1,800 500 5o -57 (2)1,800 000 54 -50 (2 1 As quenched. 2 Beyond capacity of machine.

From FIGS. 3 and 4 and Tables V and VI, it is shown that applicantsalloy exhibits good deep hardening characteristics (FIGS. 3 and 4 andTable V) and good impact resistance (Table VI), in addition to unusuallygood dimensional stability (FIGS. l and 2 and Tables II, III and IV).

The following data shown by Table VII also illustrates the widehardening range and line grain size of the alloy of the presentinvention. The material tested had the analysis reported as AL*123(2) inTable I above.

Samples 2 long were cut from 1" round annealed bar stock. They werehardened by holding for 5 minutes at the indicated temperatures and aircooling. Samples were fractured and Shepherd fracture grain size ratingsobtained. Hardness measurements were yobtained as quenched and aftertempering cumulatively at tempera- 7 tlires of 300 F., 400 lF., 500 F.,600 F., 700 F., 800 F., 900 F. and 1000 `F. Results are tabulated below:

ability in the annealed condition and good dimensional stability duringheat treatment.

TABLE VII Hardness, Rockwell C Hardeningtempera- Shepherd ture, F.fracture Tempering temperatures grain size As Quenched 300c F 400 F. 500F 600 F. 700 F. 800 F. 900 F 1,0)0 F.

3 25.5 2 .5 26.5 25 25 24 23 22 21 8 53 53. 5 53 5l 51 48. 5 45 44 41 8%59 59 57 50 53 50 47 42 9% 61 61 59 56 54 5l 47 47. 5 46 9% 63 G3 60. 558 57 54 52 50 47. 5

The above specific examples are given to illustrate the properties ofapplicants novel composition and in no way limit the scope of applicantsinvention or claims to the exact analyses set forth.

I claim:

1. An iron base alloy consisting essentially of greater than .80% andless than .90% carbon, 1.85% to 2.15% manganese, .25% to .50% silicon,.75% to 1.10% chromium, 1.20% to 1.40% molybdenum, .45 to .85% vanadium,the balance iron plus incidental impurities, said alloy Ibeingcharacterized by good deep hardening characteristics, awide hardeningrange, good impact resistance, good machinability in the annealedcondition and good dimensional stability during heat treatment.

2. An `iron base alloy consisting of greater than .80% and less than.90% carbon, 1.85% to 2.15 manganese, .25% to .50% silicon, .75% to1.10% chromium, 1.20% to 1.40% molybdenum, .45% to .85 vanadium, thebalance iron plus incidental impurities, said alloy being characterizedby good deep hardening characteristics, a wide hardening range, goodimpact resistance, good machin- 3. An iron base alloy consistingessentially of .83% to .87% carbon, 1.85% to 2.15% manganese, .25% to.50% silicon, .75% to 1.10% chromium, 1.20% to 1.40% molybdenum, .45% to.85% Vanadium, the balance iron plus incidental impurities, said alloyIbeing characterized by good deep hardening characteristics, a Widehardening range, good impact resistance, good machinability in theannealed condition and good dimensional stability during heat treatment.

4. An iron base alloy consisting of .83% to .87% car-` bon, 1.85% to2.15% manganese, .25% to .50% silicon, .75 to 1.10% chromium, 1.20% to1.40% molybdenum, .45% to .85% vanadium, the balance iron plusincidental impurities, said alloy being characterized by good deephardening characteristics, a Wide hardening range, good impactresistance, good machinability in the annealed condition and gooddimensional stability during heat treatment.

References Cited in the le of this patent UNITED STATES PATENTS1,972,524 Kinzel Sept. 4, 1934

1. AN IRON BASE ALLOY CONSISTING ESSENTIALLY OF GREATER THAN 80% ANDLESS THAN .90% CARBON, 1.85% TO 2.15% MANGANESE, .25% TO .50% SILICON,.75% TO 1.10% CHROMIUM, 1.20% TO 1.40% MOLYBDENUM, .45% TO .85%VANADIUM, THE BALANCE IRON PLUS INCIDENTAL IMPURITIES, SAID ALLOY BEINGCHARACTERIZED BY GOOD DEEP HARDENING CHARACTERISTICS, A WIDE HARDENINGRANGE, GOOD IMPACT RESISTANCE, GOOD MACHINABILITY IN THE ANNEALEDCONDITION AND GOOD DIMENSIONAL STABILITY DURING HEAT TREATMENT.